Microcystins (or cyanoginosins) are a class of toxins produced by certain freshwater cyanobacteria; primarily Microcystis aeruginosa but also other Microcystis, as well as members of the Planktothrix, Anabaena, Oscillatoria and Nostoc genera. Over 50 different microcystins have been discovered so far, of which Microcystin-LR is the most common. Chemically they are cyclic heptapeptides produced through nonribosomal peptide synthases.